1. Field of the Invention
The present invention relates to a mask pattern suitable for an exposure apparatus for exposing a photosensitive substrate with a mask pattern used in a photolithography step of manufacturing, e.g., semiconductor elements, and a photosensitive substrate alignment technique.
2. Related Background Art
In a photolithography step of manufacturing microdevices such as a semiconductor device, a liquid crystal display element, an image pickup element (CCD), and a thin film magnetic head, a projection exposure apparatus is used. In this apparatus, the image of a photomask or reticle (to be referred to as a "reticle" hereinafter) on which a transfer pattern is formed is projected on a photosensitive substrate (e.g., a wafer or a glass plate coated with a photoresist) through a projection optical system.
In the projection exposure apparatus of this type, the reticle and the wafer must be aligned to each other with a high precision prior to an exposure process. To perform this alignment, a position detection mark (alignment mark) is formed (transferred by exposure) on the wafer in the first photolithography step, and an accurate position of the wafer (or a circuit pattern on the wafer) can be detected by detecting the position of this alignment mark.
There are three major methods of detecting the position of a mark on a wafer. In the first method, an mark image is picked up to detect its position by image processing. The second method is a laser beam scanning scheme in which a grating-like alignment mark with a pitch in a direction perpendicular to the measurement direction is scanned relative to a sheet beam to detect the position of the mark based on a change in the intensity of a beam scattered or diffracted by this mark. The third method is called a "grating alignment" method using a grating-like alignment mark with a pitch in the measurement direction. This method is further subdivided into several methods in accordance with the arrangements of optical systems.
In the first arrangement, two laser beams are incident on an entire alignment mark from the directions of particular diffracted orders to detect the mark position based on a change in the amount of the beam diffracted by the mark. This method includes a heterodyning scheme and a homodyning scheme. In the former scheme, a slight frequency difference is generated between two laser beams to move interference fringes formed by this difference at a constant speed. In the latter scheme, an alignment mark is moved with respect to still interference fringes formed by two laser beams having no frequency difference.
In the second arrangement, one laser beam is incident on an entire alignment mark, two beams diffracted by this mark are condensed on a "reference grating" and formed into an image, and the alignment mark and the reference grating are scanned relative to each other to detect the position of the mark based on a change in the amount of a transmitted beam or a reflected beam by the reference grating.
Although a position detection optical system can also employ an off-axis scheme in which an exclusive microscope is arranged in addition to a projection optical system, a TTL (Through The Lens) scheme using a projection optical system itself as a position detection optical system is generally excellent in detection stability. Normally, a projection optical system is optimumly designed for an exposure wavelength (quasimonochromatic ultraviolet ray) and has a large chromatic aberration with respect to a position detection beam (its wavelength is normally 500 nm or more). For this reason, in the TTL scheme, the position detection beam is limited to a monochromatic beam or a quasimonochromatic beam. To the contrary, a method in which position detection optical systems are separately arranged for detection beams having a plurality of wavelengths to perform position detection with the plurality of wavelengths even in the TTL scheme, and a method in which an optical system for correcting a chromatic aberration of a projection optical system is arranged, and a detection beam with a certain wavelength width is used have been attempted.
An alignment mark formed on the surface of, e.g., a wafer to be used in the above position detection methods is generally a corrugated mark with a difference in depth. This corrugated mark is slightly asymmetric due to processes such as etching and sputtering in a photolithography step, and coating nonuniformity of a photoresist. This asymmetry decreases the position detection precision.
Of these position detection schemes, particularly in the TTL scheme, the temporal coherence length of a detection beam is large because the detection beam is a monochromatic beam or a quasimonochromatic beam whose wavelength width is not so large. Therefore, in the asymmetric corrugated mark, the asymmetry in the direction of mark height (depth) adversely affects the position detection precision.
When an exclusive microscope is used, although a chromatic aberration is not limited in the wavelength band of a detection beam, its wavelength band is substantially limited within the range of 500 nm to 1,500 nm to prevent of exposure of a photoresist on a wafer and by the transmittance of optical glass. For this reason, the temporal coherence length is 1 .mu.m or more, which becomes 1/1.66=0.60 .mu.m in a photoresist having a refractive index of, e.g., 1.66. As for an alignment mark whose difference in depth at a recessed or projecting portion is 0.60/2=0.30 .mu.m (the value is divided by 2 because the detection beam is reflected to reciprocate), the asymmetry in the direction of height (depth) also adversely affects the position detection precision even with the exclusive microscope.
When a detection beam having a wide wavelength is used in the TTL scheme, the precision is increased by averaging the wavelengths in an alignment mark with a difference in depth to a certain extent. In an alignment mark with a difference in depth smaller than the coherence length, however, the precision cannot be increased unlike in the above alignment mark.
If the position detection precision is decreased due to a difference in depth at a recessed or projecting portion and coating nonuniformity of a photoresist, the alignment (overlapping) precision between a reticle and a wafer is decreased in a photolithography step of manufacturing microdevices such as a semiconductor device, and particular in an exposure step. A microdevice with predetermined characteristics cannot be attained.